This invention relates to the field of well logging and, more particularly, to a method and apparatus for determining nuclear magnetic resonance logging characteristics of earth formations surrounding a borehole, either during or after the drilling of the borehole.
In the evaluation of earth boreholes drilled in earth formations to produce hydrocarbons, determination of the porosity of the formations is considered essential for decision making. Nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) provides a means of measuring total and producible porosity of earth formations. In certain conditions NMR well logging can provide important information on the pore size of formation rock and on the type of fluid contained therein. Measurement of nuclear resonance requires a static magnetic field {overscore (B)}0 and a radio frequency (RF) magnetic field in the earth formation that is being probed. [As used herein, an RF field generally has a frequency in the range 2 KHz to 10 MHz.] Atomic nuclei with a nonzero nuclear magnetic moment and spin angular momentum precess about the static field {overscore (B)}0 with an angular frequency "PHgr"0=xcex3B0 when perturbed from their thermal equilibrium. The constant xcex3 is the gyromagnetic ratio of the resonating nucleus, most commonly the hydrogen nucleus. For hydrogen nuclei, the gyromagnetic ratio is 2.675198775xc3x97108 radian/second/Tesla. To manipulate the spin state of the particles, for example, to perturb the thermal equilibrium, a radio frequency (RF) magnetic field {overscore (B)}1 is needed. The frequency of the RF field {overscore (B)}1 should be close to "PHgr"0 and substantially perpendicular to the static field {overscore (B)}0 in the region of investigation. Magnetic resonance is observed by detecting the oscillating magnetic field produced by the precession of the spins. Typically, but not necessarily, the same coil that produces the RF field {overscore (B)}1 is used for detection. In pulsed-NMR, repeated pulses are applied to the coil and spin-echoes are detected in between the transmitted pulses. Reference can be made, for example, to U.S. Pat. Nos. 5,376,884, 5,055,788, 5,055,787, 5,023,551, 4,933,638, and 4,350,955 with regard to known nuclear magnetic resonance logging techniques.
In logging-while-drilling, the measurement apparatus is mounted on a drill collar. Drill collars are long, tubular pieces of a strong material, typically nonmagnetic stainless-steel. Drill collars and drill pipes transmit the torque from the surface apparatus to the drill bit. During drilling, the drill collars typically rotate about their axes, which are substantially aligned with the axis of the borehole. The rates of rotation of the drill collars and the drill bit are the same in rotary drilling, and can be different if a downhole mud motor is used. In either case, the drill collar is subject to rotation. For logging-while-drilling NMR logging, the magnitudes of {overscore (B)}0, {overscore (B)}1, and the angle between them should be substantially invariant of the rotation angle in the region of investigation. This does not preclude the possibility that the directions of {overscore (B)}0 and {overscore (B)}1 may depend on the rotation angle. The foregoing invariance is required because magnetic resonance measurements take on the order of 0.01 to 1 seconds during which the drill collar may rotate by a substantial angle. Consistent preparation and measurement of spin states are not possible without the rotational invariance.
Directional drilling involves the drilling of a well bore along a deviated course in order to reach a target region at a particular vertical and horizontal distance from the original surface location. Directional drilling is employed, for example, to obtain an appropriate well bore trajectory into an oil producing formation bed (or xe2x80x9cpay zonexe2x80x9d) and then drill substantially within the pay zone. A horizontally drilled well can greatly increase the borehole volume in the pay zone with attendant increase in oil production. Recent advances in directional drilling equipment and techniques have greatly improved the accuracy with which drilling paths can be directed.
Nuclear magnetic resonance logging systems have previously been proposed for logging-while-drilling applications. If an NMR logging device of a logging-while-drilling system has an axially symmetric response, the NMR characteristics measured by the logging device will tend to average the signals received circumferentially from the formations. For example, when drilling a near-horizontal well along the boundary between two formation beds with dissimilar producible porosities, such a logging device would give indication of an intermediate porosity. It would be very advantageous to be able to use NMR to better delineate the presence, locations, and characteristics of the formation beds in this type of a situation.
It is among the objects of the present invention to address limitations of the prior art with regard to nuclear magnetic resonance logging techniques and apparatus.
The invention described in the copending parent application hereof, U.S. application Ser. No. 08/880,343, provides the capability of azimuthally resolved nuclear magnetic resonance logging. That invention and the invention hereof can both be used in so-called wireline logging, but the inventions are particularly advantageous in achieving azimuthally resolved NMR logging-while-drilling measurements.
A form of the invention set forth in said copending U.S. application Ser. No. 08/880,343 is directed to an apparatus and method for determining a nuclear magnetic resonance property of formations surrounding a borehole while drilling the borehole with a rotating drill bit on a drill string. An embodiment of the method of that invention includes the following steps: providing a logging device in the drill string, the logging device being rotatable with the drill string or a portion of the drill string, the logging device having a rotational axis; producing a static magnetic field and an RF magnetic field at the logging device, the static and RF magnetic fields having mutually orthogonal components in an investigation region in the formations surrounding the logging device, the magnitudes of the static and RF magnetic fields in the investigation region being substantially rotationally invariant as the logging device rotates around its axis; receiving nuclear magnetic resonance spin echoes at at least one circumferential sector on the logging device; and determining a nuclear magnetic resonance property of the formations, for different portions of the investigation region, from the received nuclear magnetic resonance spin echoes. [It will be understood that the static and RF magnetic fields are defined as having xe2x80x9cmutually orthogonal componentsxe2x80x9d if they are not parallel. Typically, but not necessarily, the static and RF magnetic fields will be substantially perpendicular in the investigation region.]
In another form of the invention set forth in said copending Application, the receiving of nuclear magnetic resonance spin echoes is implemented at a plurality of different circumferential sectors on the logging device and comprises providing a plurality of arcuate receiver segments around the logging device and detecting nuclear magnetic resonance spin echoes in signals received by the individual receiver segments.
In embodiments hereof, in order to make an azimuthally-resolved NMR measurement, the receiving radio-frequency antenna has a non-axisymmetric response pattern. The transmitting (pulsing) antenna can be either the same non-axisymmetric antenna, or a separate non-axisymmetric antenna, or a separate axisymmetric antenna. If the azimuthal measurement is to be performed while the tool is rotating (as is typically the case in MWD), the static magnetic field ({overscore (B)}0) should be axisymmetric, at least in terms of its magnitude. In certain embodiments hereof, the rf antennas employ axial currents (parallel to the tool and wellbore axis), which excite an azimuthally-oriented rf magnetic field ({overscore (B)}1). For this situation, the static magnetic field ({overscore (B)}0) should be either radial or axial in its orientation, so as to be approximately perpendicular to the azimuthal {overscore (B)}1 field excited by the rf antenna, as is desirable for efficient NMR signal generation and reception. In another embodiment, the rf antenna excites a radially-oriented rf magnetic field. In this case, the static magnetic field should be either axial, transverse, or azimuthal in its orientation. In embodiments of the invention to be described, a region of generally uniform static field magnitude and polarization produced in the formations is relatively long in axial extent, and an advantage is that the rf antenna used to obtain azimuthally resolved measurements can also be made relatively long in the axial direction, thereby increasing the volume of spins ultimately sensed by the antenna and increasing signal-to-noise ratio. This increase tends to offset the decrease in the volume of spins that are detected when the azimuthal range of investigation is limited to a sector that is a fraction of a full circumference.
A form of the invention is a method for determining a nuclear magnetic resonance property of formations surrounding a borehole during a drilling operation in the borehole with a drill string, comprising the following steps: providing a logging device in the drill string, the logging device having a longitudinal axis; producing, from said logging device, a static magnetic field and an rf magnetic field in the formations; and receiving nuclear magnetic resonance signals from an investigation region of the formations at an antenna having a response pattern that is non-axisymmetric, said response pattern having an azimuthal polarization in the investigation region.
In accordance with an embodiment of apparatus in accordance with the invention, there is disclosed an apparatus for determining a nuclear magnetic resonance property of formations surrounding a borehole which comprises: a logging device moveable through the borehole; means in said logging device for producing a static magnetic field in the formations; and antenna means in said logging device for producing an rf magnetic field in the formations, and for detecting nuclear magnetic resonance signals from the formations, the antenna means including a plurality of spaced apart generally cylindrical arc-shaped conductors, and means coupled across the arc-shaped conductors for detecting signals induced in the conductors. In a preferred embodiment of this form of the invention, the logging device has a longitudinal axis, and the cylindrical arcs of the conductors are concentric with said axis. In this embodiment, the means for producing an rf magnetic field in said formations is operative to generate current flowing in adjacent conductors in opposing axial directions and the means for detecting signals is operative to detect currents flowing in adjacent conductors in opposing axial directions.
In another embodiment of the invention, the antenna means includes at least one multi-turn current loop having an axis that is substantially perpendicular to the longitudinal axis of the logging device, and means coupled with said current loop for detecting signals induced in said current loop. In a preferred form of this embodiment, the at least one multi-turn current loop is formed in a generally cylindrical arc shape, said arc being centered on a line oriented in the direction of the longitudinal axis.
In embodiments of the invention, an antenna is low profile and can be formed in an outer groove in the drill collar without necessarily reducing the inner diameter of the drill collar, which is ordinarily done to increase strength in a region of drill collar where the outer diameter has been recessed to provide an antenna. [The reduction in the bore size of the drill collar is preferably avoided, if possible, as it requires extra machining of the drill collar and contributes to constriction of mud flow.] In accordance with these embodiments there is disclosed an apparatus for nuclear magnetic resonance logging that is mountable in a drill string for logging of formations surrounding a borehole, comprising: a tubular drill collar having a generally cylindrical inner surface having an inner diameter and a generally cylindrical outer surface having an outer diameter; first means in the drill collar for producing a first magnetic field; second means in the drill collar for producing a second magnetic field; and means in the drill collar for receiving nuclear magnetic resonance signals from an investigation region in the formations; the second means comprising an antenna disposed in a recess spanning an axial extent in the outer cylindrical surface, the outer surface of said drill collar having a diameter that is reduced from the outer diameter over the axial extent of the recess, and the inner surface of the drill collar having a diameter that is not reduced from the inner diameter over the axial extent of the recess. In a form of this embodiment, the antenna comprises at least one generally cylindrical arc-shaped conductor plate in the recess. In another form of this embodiment, the antenna comprises at least one multi-turn loop formed in a generally cylindrical arc shape in the recess.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.